NORMAN, Okla. (AP)  Testimony has begun in the trial of a man accused of beheading a co-worker at a food plant in Oklahoma.

Alton Nolen is charged with first-degree murder in the September 2014 beheading of 54-year-old Colleen Hufford at Vaughan Foods in Moore, Oklahoma. Nolen is also charged with assault and battery with a deadly weapon for allegedly attempting to behead a second co-worker. Prosecutors are seeking the death penalty.

Defense attorneys say the 33-year-old Nolen is mentally ill and want jurors to find him not guilty by reason of insanity.

But Cleveland County District Attorney Greg Mashburn told jurors in his opening statement on Thursday that Nolen carried out the attack and knew what he was doing was wrong.